Conventional microlithographic exposure systems are summarized below in the context of such systems as used for performing exposures using extreme ultraviolet (“EUV”) light, for example. Certain aspects of a conventional system of this type are shown schematically in FIG. 13, in which the depicted system includes an EUV source 101, an illumination-optical system 103 that irradiates a beam 100 of EUV light (λ=13.4 nm) from the EUV source 101 to a reflective pattern-defining reticle 102. “Patterned” EUV light (carrying an aerial image of the pattern portion illuminated by the beam 100) passes through a projection-optical system 105 that projects the aerial image onto the resist-coated surface of a wafer 104 or other suitable lithographic substrate. During this exposure the reticle 102 is held on a reticle stage 106, and the substrate 104 is held on a “wafer stage” 107, and exposure of the substrate 104 results in “transfer” of the pattern, defined on the reticle 102, onto the substrate 104.
The projection-optical system 105 typically comprises multiple (e.g., two, four, or six) multilayer-coated reflective mirrors (not detailed). As the projection-optical system 105 projects the aerial image onto the substrate surface, the image is demagnified or “reduced,” usually by a factor of 5 (i.e., the projection-optical system exhibits a “demagnification ratio” of 1/5). The projection-optical system 105 typically has an annular-shaped optical field for exposure, for example 2-mm wide and 30-mm long as projected onto the wafer 104. Each of the reflective multilayer-coated mirrors of the projection-optical system 105 typically has an aspherical reflective surface. So as to be highly reflective to incident EUV radiation of λ=13.4 nm, the reflective surface of each mirror has a multilayer-film coating such as alternating layers of Mo and Si. During an actual exposure, the reticle 102 and wafer 104 are scanningly moved by their respective stages 106, 107 at synchronous velocities (according to the demagnification ratio). For example, under these conditions the wafer 104 is scanned at a velocity that is 1/5 the scanning velocity of the reticle 102. By performing exposure in a scanning manner in this way, it is possible to transfer a large pattern that extends over an area that is larger than the width of the optical field of the projection-optical system 105.
FIG. 14 shows in greater detail an exemplary conventional EUV optical column 110 such as a type that would be used in an EUV microlithography system. The particular optical column 110 of FIG. 14 comprises two reflective mirrors (more generally termed “optical elements”) 111, 112 and their respective mountings 116, 117. The optical column 110 also comprises a column main unit 110a and a flange unit 110b that desirably are made of a low-thermal-expansion material such as invar so as not to be exhibit excessive thermal deformation. The mounting 116 for the mirror 111 includes a position-adjustment mechanism 115 (e.g., a piezoelectric motor) mounted on the upstream-facing surface of the flange unit 110b. The position-adjustment mechanism 115 allows the position of the mirror 111 to be adjusted relative to the flange unit 110b during and after assembly of the optical column 110. The mounting 117 for the mirror 112 is mounted to the downstream-facing surface (lower surface in the figure) of the flange unit 110b. Respective voids 111a, 112a are defined in each of the mirrors 111, 112. An EUV beam 100 reflected from the surface of a reticle (not shown, but situated upstream of the depicted optical column 110) propagates to the upper surface of the mirror 112 through the void 111a in the mirror 111. Light of the EUV beam 100 reflected from the upper surface of the mirror 112 propagates to the lower surface of the mirror 111, from which the EUV beam 100 is reflected downward through the void 112a in the mirror 112 to the substrate (not shown but situated on an image plane, just downstream of the depicted optical column 110, at which the beam 100 converges).
So as to be reflective to incident EUV radiation, the respective surface (which is aspherical) of each mirror 111, 112 of the optical column 110 of FIG. 14 has a surficial Mo/Si multilayer coating. The depicted optical system has a numerical aperture (NA) of 0.3 and exhibits a wavefront aberration of no greater than 1 nm (RMS). To achieve such demanding performance, the respective aspherical surface of each mirror 111, 112 is formed with extremely high accuracy before the multilayer coating is applied. In addition, the multilayer coatings are applied with extremely high accuracy, and each mirror 111, 112 is mounted in the optical column 110 with extremely high accuracy, preferably in a manner allowing independent adjustment of the mirrors while in the optical column.
This adjustable placement of each mirror 111, 112 in the optical column 110 is achieved by the respective mountings 116, 117. It is important that the mountings 116, 117 be capable of holding the respective mirrors 111, 112 without causing any deformation of the mirrors 111, 112. It also is important that the mountings 116, 117 be capable of preventing positional shifts of the respective mirrors 111, 112 while the mirrors are mounted in the optical column 110. Unfortunately, conventional mountings for optical elements do not exhibit satisfactory performance in these regards, especially in optical columns intended for extremely demanding use, such as in EUV optical columns for use in EUV microlithography systems.